The present invention relates to method and apparatus for the production of semiconductor ribbons from a melt and more particularly to improved apparatus and method in which rate of heat flow from the molten material into the surface of a rotating wheel is controlled.
A reference known to the Applicants and believed to be relevant to the present invention is U.S. Pat. No. 3,605,863 issued to King on Sept. 20, 1971. This patent is hereby incorporated by reference for its general teachings of apparatus and methods for formation of ribbon materials which are generally applicable to formation of semiconductor ribbons.
It is generally recognized that a major portion of the cost of production of solar cells is the cost of the original high-purity silicon and its preparation into wafers of suitable size and shape. A standard process involves the growing of monocrystalline boules using the Czochralski technique and then mechanically sawing the boules into thin slices or wafers. Half of the original silicon material is typically lost in a sawing process. Photovoltaic cells have also been made from polycrystalline silicon after it was poured into essentially brick-shaped molds and then sawed into square slices. While such processing of polycrystalline material is less expensive than growth of singlecrystal material, the resulting cells have lower efficiency and half of the material is still lost in the sawing process. Much effort has been made to develop apparatus and techniques for producing wafers or ribbons of silicon directly from the molten state. The above-referenced King patent illustrates one type of apparatus which has resulted from such efforts.
The basic method illustrated by the King patent includes the contacting of a molten body of material with the surface of a moving drum or wheel. The surface of the wheel is cool relative to the molten mass and by cooling a portion thereof pulls or drags off a film or ribbon of the material in a solidified or partially-solidified form. In practice of this type of method the control of heat flow from the molten material into the cool wheel surface is critical. Various parameters have been adjusted in efforts to obtain commercially practical methods of operation. The temperature of the molten silicon, the temperature of cooling fluid in the wheel and the rate of rotation of the wheel all have major effects on the heat-transfer process. Minor changes in these parameters will in many cases totally disrupt the ribbon-production process. In refining the ribbon-making process it has been found desirable to remove the ribbon from the wheel while it is still in a somewhat plastic state before complete solidification. This requires even closer control of heat extraction from the silicon as it contacts the rotating wheel surface.
In view of the quantity of heat which must be removed from the molten silicon in the ribbon-making process and the tempertures which are involved, it is necessary that the cool drum or wheel be structurally strong and have a large heat capacity. The wheel rim is typically made from a metal such as stainless steel having about one-inch thickness. The inner surface of the drum is typically cooled with a fairly large quantity of circulating fluid such as water which is able to remove heat rapidly. This massive nature of the cooling wheel as compared to the relatively thin, 0.01 to 0.02 inch thick, ribbon makes fine-tuning of the heat-extraction process by techniques which have heretofore been applied more difficult.